First, thank you for the detailed explanation of your problem, including code and log files (I don't often get that kind of detail and it is very helpful).
That said, I'm not sure I'm going to be able to help you, at least not right away. Due to GAE being a non-standard Java environment, there's been a bit of a challenge getting Spring Social to work in GAE. The whitelist (or more accurately, the things not on the whitelist) are one source of the problem. And considering the Spring Social leverages other libraries to do its work, that means that not only must Spring Social stick to the GAE whitelist, but those other libraries must also be confined to the whitelist.
But there were a few bugs in GAE that also prevented me from getting Spring Social to work 100% in GAE. I came close on one occasion, but due to a bug in GAE I couldn't get ProviderSignInController to fully work.
https://jira.springsource.org/browse/SOCIAL-73 is one issue that tracks some of this effort. But it has been well over a year since I've tried this again.
But I wanted to see if I could at least figure out where this is failing in Spring Social to give some idea if there's a workaround. Unfortunately, line 177 in the latest ProviderSignInController code is a JavaDoc comment...clearly not the issue. So, I must ask: Which version of Spring Social are you using?
Spring Social Project Lead